The present invention relates to hydraulic cylinders and more particularly to a composite structure for hydraulic cylinders consisting essentially of a plastic housing molded around a metallic sleeve.
Hydraulic cylinders, such as hydraulic brake master cylinders and master and slave cylinders for hydraulic apparatus controlling the operation of motor vehicle mechanical clutches, have been made in the past entirely of impact and burst resistant plastics or, preferably, of a housing of plastic provided with an internal metallic tubular member or sleeve defining the bore of the cylinder in which a piston reciprocates. The cylinder housing, when it is molded of plastic, is provided with a tapered bore because of the requirement of providing internal cavities of a molding with a certain amount of draft to facilitate unmolding, as is well known in the art. The bore of the plastic housing is subsequently provided with a metallic sleeve made of steel, aluminum or other metal or metal alloy which provides a smooth and substantially cylindrical surface in which the piston, including the piston seal, is slidably disposed. Such a structure requires that means be provided for retaining the metallic sleeve in the bore of the plastic housing to prevent longitudinal motion of the sleeve relative to the housing and to ensure appropriate sealing from the ambient, principally at the pressure end of the cylinder. In addition, in view of progressive increasing clearance from one end of the cylinder to the other between the peripheral cylindrical surface of the metallic sleeve and the tapered, due to the required draft, internal surface of the plastic housing, additional seal means are provided between the metallic sleeve and the bore of the housing, not only at the pressure end of the cylinder, but also at the open end of the cylinder in order to prevent leakage of hydraulic fluid from the interior of the cylinder to the ambient, and a retainer is installed at the open end of the cylinder for preventing the piston from escaping from the cylinder bore.
Stringent sealing requirements are particularly prevalent in master cylinder structures. Hydraulic master cylinders are connected to a supply of hydraulic fluid in a hydraulic fluid reservoir, either formed integrally with the cylinder housing or made separate from the cylinder housing and connected thereto by an appropriate conduit. Hydraulic fluid is supplied from a reservoir to the master cylinder pressure chamber through appropriate ports through the wall of the cylinder such that loss of hydraulic fluid from the system may be compensated for and excess fluid, due to heat expansion for example, may be returned to the reservoir of fluid. The port or ports through the wall of the cylinder are appropriately masked by the piston seal at the beginning of the piston working stroke such as to cut off the reservoir from the cylinder pressure chamber during operation. Because the ports are formed through the wall of the cylinder housing and through the wall of the metallic sleeve or liner, and, because in prior art structures a narrow space exists between the peripheral surface of the sleeve or liner and the internal surface of the housing due to the fact that the internal bore in the housing is frusto-conical rather than perfectly cylindrical and the peripheral surface of the sleeve or liner is circularly cylindrical and of constant diameter from end to end, the entire narrow annular space between the bore of the cylinder and the peripheral surface or liner is filled with hydraulic fluid, thus requiring additional sealing means to be provided to prevent escape of hydraulic at the ends of the cylinder.